The present disclosure deals with processes and apparatus for calculating instability within a power distribution system and optionally correcting the instability.
Power distribution systems, also known as electrical power grids, are used to transmit power from power generators to consumers. Over time, power distribution systems have become increasingly complex and more difficult to govern, resulting in their increased instability. This is evidenced by the Northeast Blackout of 2003, as an example.
In addition to the general growth in the complexity of power distribution systems to accommodate greater consumer demand and adapt to growing and changing topologies, several factors to some extent—and are predicted to increasingly—contribute to increased instability in power distribution systems. One such factor is the deregulation of the electrical utility industry. The deregulation of the industry has led to an increased number of power brokers that each services relatively fewer consumers as compared to prior regulatory schemes. This fragmentation of the market leads to increased instability of the electrical power grid by making the transfer of power through different points in the grid less predictable due to power brokers vying for customers and routing power through different pathways in attempting to optimize the price of power delivered to consumers.
Another factor that has significantly impacted the stability of the power grid has been the development of smart appliances and timers. These appliances can be programmed to operate at different times of the day than was previously feasible. This is sometimes to take advantage of lower electricity costs during off hours and sometimes to operate when the users are asleep. The operation of these appliances has made the demand on the power distribution systems less predictable by decreasing demand during peak hours and increasing demand during off hours. Yet another factor is the increased supply of renewable energy in power distribution systems. Many sources of renewable energy, and in particular wind and solar power, cannot provide consistent power generation when compared to coal, gas, and nuclear power plants, thereby creating fluctuations in the supply of power.
Thus, power distribution systems are increasingly being subjected to unpredictable fluctuations in both supply and demand which leads to increased instability.
Instability in power distribution system can come from a variety of sources. As previously discussed, differences in consumer demand and fluctuations in supply due to the use of renewable resources are increasing sources of instability. Additionally, faults in the power distribution system are a common source of instability. Electrical generators used in these systems are generally synchronous and physically large, resulting in several phenomena when they are in operation. First, the generators can act as a generator or as a load depending on the direction of flow of electricity between the rest of the power distribution system and the generator. This phenomenon commonly occurs depending in part on the phase difference of the generator and the rest of the power distribution system. A power distribution system as a whole can generate a much greater amount of power than a single generator. Therefore, if there is a phase difference between one generator and the rest of the system, the generator will attempt to correct itself such that it is synchronized (in phase) with the rest of the power distribution system. The power used to synchronize the generator can flow from the power distribution system and through the generator and therefore heat the wire windings of the generator. If the power required for synchronization is too great, it can lead to catastrophic failures and permanent damage to the generator. Similar results can also occur if any two sections of a power distribution system are significantly out of phase and yet remain interconnected.
Therefore, it is important to synchronize each generator with the rest of the power distribution system. However, this is an over simplified model in that phase of the power distribution system is not perfectly consistent throughout the system. The phase can be altered through the transfer of power through transmission lines, the amount of loading of the system by customer appliances/devices, the changes in the reactance (capacitive or inductive) of the loads, through phase shifting transformers, and through other means. Additionally, the precise phase of an operating generator is currently difficult to ascertain. Although there are plans to directly read the phase using an encoder or other such device, currently the phase of a generator is generally estimated by sensing the phase of electricity in close proximity to the generator.
The relatively large size of generator rotors also makes it difficult to adjust the phase of the generator quickly enough to respond to fluctuations in phase because the generators have a relatively large amount of inertia as they spin. Generally, other mechanisms are therefore used to prevent or correct instability in the system. These can include power shedding, fault clearing, shunt reactors, or other active devices to alter the phase of electricity at a point in the power distribution system.
However, the correct mechanism(s) should be enabled and at the correct time and for the correct duration in order to reduce system instability. One method that has been found to aid in prediction of system instability is to calculate the center of inertia of the system. The center of inertia is defined by the equation
            δ      C        =                            ∑                      i            =            1                    N                ⁢                              δ            j            i                    ⁢                      P            j            i                                                ∑                      j            =            1                    N                ⁢                  P          j          i                      ,where δc is the center of inertia, δji is the internal generator rotor angle of j generator in area i, and Pji is the power generated by j generator in area i, and N is the number of generators in area i. The internal generator rotor angle is generally estimated to be similar to the phase of the electricity that the generator outputs which is sampled by a device known as a PMU (Phase Measurement Unit). However, enhanced PMUs have recently been developed and are known as synchrophasors.
A synchrophasor is a device attached to the power distribution system that senses amplitude, frequency, and phase information of electricity flowing through the power distribution system where the synchrophasor is located. This information together is generally represented by a phasor. The information can then be relayed to a remote site for further analysis. Additionally, the synchrophasors are synchronized to a common clock such that the phasor information can be related to the time at which the information was sensed. The common clocking of the synchrophasors further enables phase differences throughout the power distribution system to be more precisely ascertained. However, algorithms to efficiently use this information are still under development and different manufacturers of synchrophasors can have different tolerances or standards. Therefore, synchrophasors are a step in the right direction, but do not always provide consistent and therefore useable information.
The center of inertia is useful in estimating the phase of electricity within all or a portion of a power distribution system. Differences between centers of inertia between two areas of the power distribution system or between the whole power distribution system and a portion of the power distribution system are indicative of instability within the system. Although the center of inertia equation is relatively simple, it must be understood that its application to a modern power distribution system is difficult. A modern power distribution system is in constant fluctuation due to changes in supply, demand, and faults that occur within the system. Additionally, various interconnections between different power distribution systems can exist and/or be switched on or off adding even more complexity to the calculation. Instability in the system can lead to power loss due to system component failures or overcompensation via corrective actions.
It is desirable for the center of inertia to be computed in a relatively short interval for it to be useful for detecting—and even more so for correcting—instabilities within a power distribution system. Currently used calculations for the center of inertia rely on phase information from disparate synchrophasors to be aggregated by a central processor for calculation of the center of inertia. The total time to calculate the center of inertia includes the time to transmit the phase information in addition to the time required to calculate the center of inertia. Response to detected instabilities also requires additional time in order to command active component(s) of the system to correct the instability. Systems using such a topology are therefore not ideal for the detection and reacting to instabilities within a power distribution system.
Therefore, improving the instability detecting and correcting mechanisms used in power distribution systems is of utmost importance to deliver consistent power to consumers and to prevent damage to the power infrastructure. Thus, there is a need for improvement in this field.